GEOLOGIC MAP OF THE SHELL RESERVOIR QUADRANGLE, CENTRAL BIGHORN MOUNTAIN, WYOMING

The Bighorn Mountains in North Central Wyoming represent a substantial geologic record of Laramide exhumation between the Cretaceous and Eocene epochs of the North American geologic setting; however, a large-scale map of the region is still lacking. This study developed a detailed (1:24,000) geologic map of the Shell Reservoir Quadrangle of Bighorn Mountain by combining field data with laboratory analyses. The findings of this study will be a significant aid to the Wyoming State Geological Survey, allowing for the improvement and extension of the geologic map of the state.

The Bighorn Mountain has two groups of Archean rocks at its core: gneissic rocks in the south and felsic intrusive rocks in the north. The Shell Reservoir Quadrangle, which occupies the northern portion of Bighorn Mountain, consists primarily of intrusive rocks, with some sedimentary units in the southwest corner.

The intrusive rocks pose challenges in differentiating individual units due to their strikingly similar petrographic characteristics, because of which prior studies grouped all intrusive rocks as a single unit. Instead, this study divides these intrusive rocks into five units: alkali feldspar granite, adamellite, tonalite, granodiorite, and mafic dykes based on field map and petrographic and geochemical analyses of more than 45 rock samples. Each unit was differentiated based on color, texture, and relative percentages of major rock-forming minerals. All intrusive bodies contain significant fabric elements revealing magma flow directions during their emplacement. The Pinedale Glacial deposits cover a small portion of the central part. The sedimentary strata of this quadrangle include Flathead, Gallatin Gros Ventre, Bighorn, and Madison Formations. The intrusive units of the region intrude all sedimentary rocks except Flathead Sandstone.